fix INSTALLDIRS for newer PERL versions

[pgnd]

similar to

https://github.com/jonathanstowe/Term-Cap/issues/15 https://github.com/jonathanstowe/Term-Cap/pull/16

"INSTALLDIRS should not be set to "perl" in perl versions newer than 5.12, because site will take precedence."

i.e.,

@ https://github.com/andk/cpanpm/blob/master/Makefile.PL#L199
-	INSTALLDIRS  => 'perl',  # as it is coming with perl
+	'INSTALLDIRS'       => ($] < 5.012) ? "perl" : "site",

or similar, as appropriate

[pgnd]

filed https://github.com/andk/cpanpm/pull/178 for this.

[briandfoy]

#178 was merged, so this can be closed as completed.